1. Field of the Invention
The present invention relates to a planar array type microwave antenna for use in, for example, receiving satellite broadcasts.
2. Description of the Prior Art
In a suspended line feed type planar antenna in which a substrate is sandwiched between metal or metallized plastics plates having a number of openings forming parts of radiation elements, a circular polarized wave planar array antenna has been proposed. In this previously-proposed antenna, a pair of excitation probes which are perpendicular to each other, the number of which corresponds to the number of openings, are formed on a common plane and signals fed to the pair of excitation probes are mixed in phase within the suspended line.
Thus, the above-mentioned planar antenna can be reduced in thickness as compared with the existing one, and also its mechanical configuration can be simplified. Further, an inexpensive substrate now available on the market can be employed for high frequency use, achieving antenna gain equal to or greater than that of a planar antenna using an expensive microstrip line substrate.
The suspended line achieves such advantages in that it forms a low loss line as a circuit for feeding the planar antenna, and also in that it can be formed on an inexpensive film shaped substrate, and so on. Further, since this conventional planar antenna utilizes a circular or rectangular waveguide opening element as a radiation element, it is possible to construct an array antenna which has a small gain deviation over a relatively wide frequency range.
A so-called patch-slot array antenna has been proposed, which effectively utilizes features of the suspended line and thin radiation elements to provide high efficiency and wide bandwidth. Also, this type of array antenna can be reduced in thickness and weight (see our U.S. patent application Ser. No. 223,781 filed Jul. 25, 1988), now U.S. Pat. No. 4,087,920.
In the suspended line feed-type planar array antenna in which the substrate is sandwiched between the pair of metal or metallized plastics plates, a number of resonance type printed patch radiators are formed on the substrate at positions corresponding to openings formed through one of the metal or metallized plastics plates.
However, the planar array antenna described in U.S. Pat. No. 5,087,920 has formed around a number of resonance type printed patch radiators thereof flanges as supporting portions so that difficult cutting work cannot be avoided, which makes the efficient mass-production of the antenna impossible. Also, this makes the antenna expensive.
In order to solve the aforenoted problems, a suspended line feed type planar array antenna has been proposed (see our U.S. patent application Ser. No. 258,728), now U.S. Pat. No. 4,990,926, in which a substrate is sandwiched between an upper plate having a number of openings and a lower plate opposing the upper plate. Specifically, in this previously-proposed suspended line feed type planar array antenna, protrusions are formed on the upper and lower plates at their corresponding positions by a press-treatment and the substrate is supported by these protrusions. According to this antenna, difficult cutting work is not needed and only the simple press-treatment is required, which permits the efficient mass production. This can also make the antenna inexpensive.
FIG. 1 shows a circuit arrangement in which a plurality of circular polarized wave radiation elements are fed in phase by a suspended line, forming the array. In that case, the circular polarized wave radiation elements are such as described in U.S. Pat. No. 258,728. The solid line in FIG. 2 illustrates a portion cut through the line II--II in FIG. 1. The broken-line portion of FIG. 2 illustrates such a condition that the second metal plate 2 covers the top of the arrangement shown in FIG. 1.
A plurality of protrusions 11 are formed on the first metal plate 1 between the conductive foils 8 and the suspended lines in order to support the substrate 3. The protrusion 11 is further provided on the first metal plate 1 around the outer peripheral portion of the planar array antenna, as shown. Other portions of the first metal plate 1 form cavity portions 7. There is then the substantial risk that the outputs from the plurality of conductive foils 8 may be delivered through the same cavity portion 7 and hence the above-mentioned outputs will be coupled with each other. If, however, the spacing between the neighboring conductive foils 8 and the spacing between the upper and lower walls of the cavity portion 7 are properly selected, necessary isolation can be established, thus eliminating the above-mentioned risk of the mutual coupling. Since the electric lines of force are concentrated on the upper and lower walls of each cavity portion 7, the electric field along the substrate 3 supporting the conductive foils 8 is substantially removed, thus lowering the dielectric loss. As a result, the transmission loss of the line is reduced.
The protrusions and the cavity portions are also formed on the second metal plate 2 in correspondence with those of the first metal plate. More specifically, protrusions 12 are formed on the second metal plate 2 around the slots 5 bored therethrough and around the periphery of the feeding portion positions between the conductive foils 8 and the suspended lines to support the substrate 3, while other portions between the protrusions form the cavity portions 7.
Since the substrate 3 is uniformly supported by the protrusions 11 and 12 provided as described above, the substrate 3 can be prevented from being warped downwardly. In addition, since the top and bottom metal plates 1 and 2 are brought in face-to-face contact with the substrate 3 around the respective radiation elements, the feeding portions and so on similarly as described above, it is possible to prevent any resonance at a particular frequency from being caused.
Referring to FIG. 1, 16 radiation elements are arranged in groups of four, to provide four radiation element groups G.sub.1 to G.sub.4. A junction P, of the suspended lines in each group is displaced from the center point of the group by a length of .lambda.g/2 (.lambda.g represents the line wavelength at the center frequency). Junctions P.sub.2 and P.sub.3 in the suspended lines feeding two radiation elements in each group are connected with a displacement of each of .lambda.g/P.sub.4 from the center point between these two. Accordingly, in each group of the radiation elements, the lower-right-hand radiation element is displaced in phase from the upper-right-hand radiation element by 90 degrees, the lower-left-hand radiation element is displaced therefrom by 180 degrees and the upper-lefthand radiation element is displaced therefrom by 270 degrees, respectively, which results in the axial ratio being improved. In other words the axial ratio can be improved to be wide by varying the spatial phase and the phase of the feeding line. In view of another aspect, any two of vertically or horizontally neighboring patch radiators have slit directions 90 degrees apart from each other.
The junction P in each group and the junctions P.sub.4 to P.sub.6 in the suspended lines feeding the respective, groups are coupled to one another in such a fashion that they are distant from the feeding point 10 of a feeding portion 9 by an equal distance. That is, it is possible to obtain various kinds of directivity characteristics, by changing the feeding phase and the power distribution ratio, by changing the positions of the junction P, and the junctions P.sub.4 to P.sub.6. In other words, the feeding phase is changed by varying the distances from the feeding point 10 to the junctions P, and to the junctions P.sub.4 to P.sub.6, and the amplitude is varied by varying the impedance ratio by increasing or decreasing the thickness of the lines forming the various branches of the suspended line, whereby the directivity characteristics can be varied in a wide variety of range.
According to the method in which the substrate is supported by a number of protrusions as shown in FIG. 1, the protrusions are formed on the pair of metal plates between the conductive foils, and the patch slot type resonance print elements deposited on the substrate are coaxial with the slots and the suspended lines, so that no problem will arise in a portion where the protrusions are concentrated to some degree. However, in a portion where the protrusions are formed poorly, the substrate can not be uniformly supported at its intermediate portion. Thus, the positional displacement of the substrate occurs in the up and down direction partly in the upper to lower direction. In worst cases, the substrate is slackened. There is then the substantial risk that the printed radiation element will touch the metal plate. As a result, there is the substantial disadvantage that deterioration of antenna characteristic such as the decrease of antenna gain or the like will occur.
Furthermore, since a number of protrusions have to be formed in correspondence on the pair of plates, the number of manufacturing-process for manufacturing the plates is increased and the productivity is relatively poor.
Therefore, in a suspended line feed type planar antenna in which a substrate is sandwiched between an upper plate having a number of openings and a lower plate opposing to the upper plate, spacers or distance pieces having a number of corresponding openings are provided between the upper plate and the substrate and between the substrate and the lower plate, respectively thereby supporting the substrate. Thus, the substrate can be positively supported at the intermediate portion between the upper and lower plates with a uniform distance therebetween. As a result, the protrusions formed on the upper and lower plates can be reduced considerably, which makes the manufacturing process of the upper and lower plates simple and which can increase the productivity (see Japanese Patent Application No. 63-199513).
FIG. 3 shows in cross section a structure of planar array antenna described in Japanese Patent Application No. 63-199513. In FIG. 3, reference numeral 20 designates a rear cover, 21 a lower plate, 22 a distance piece or spacer, 23 a film substrate on which a number of resonance type printed patch radiators (radiation elements) 23' are printed, 24 a distance piece or spacer, 25 an upper plate, 26 a support cushion made of low foaming styrol and 27 a radome. In that case, the rear cover 20 is 3 mm in thickness, the upper and lower plates 21, 25 and distance pieces 22, 24 are 1 mm in thickness, respectively, the support cushion 26 is 12 to 14 mm in thickness, and the radome 27 is 1 mm in thickness. The entire thickness of this planar array antenna is about 20 to 22 mm.
This previously-proposed planar array antenna shown in FIG. 3 can not avoid the following shortcomings and disadvantages:
(1) Since the distance between the radiation element 23, and the lower plate 21 provided as the ground plate is 1 mm, the change of element impedance and the ratio in which the operational gain is changed are made large due to slackening of the film substrate 23. PA1 (2) Since the distance between the lower plate 21 and the upper plate 25 is 2 mm, the feed line loss is large. For example, when the line width was 1.5 mm at the frequency of 12 GHz and the characteristic impedance Z.sub.0 of line was selected to be 76.OMEGA., feed line loss was 1.6 to 1.8 dB/m. PA1 (3) The element gain is small (about 6.5 dB). PA1 (4) The impedance matching band width of elements is narrow. PA1 (5) Since the resonance type printed patch radiator is of the type for feeding one feed point, the circularly polarized wave band is narrow and the pair of four elements must be fed with a phase difference therebetween. PA1 (6) Because of the disadvantages (4) and (5), the excitation balance of elements can not be made without difficulty.